Synthesis of Well-defined Solid Catalysts by Surface Organometallic Chemistry - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 9

d n 9 r 4 n g | 1

Scheme 1.4 Grafting reaction of trisneopentyl neopentylidene tantalum on a silica
surface.

contain two types of ligands which could lead, a priori, to two types of sur-
face complexes. However, only one surface species is obtained, for example
Si-O-Ta[CH 2 -C(CH 3 ) 3 ] 2 [ ¼ CH-C(CH 3 ) 3 )] in the case of tantalum. 45,50 Two
mechanisms can be proposed for the grafting reaction: (i) selective reaction
of the metal alkyl moiety as for the alkyl complexes or (ii) addition of the
oxygen atom on the carbene moiety followed by a-H abstraction and neo-
pentane evolution (Scheme 1.4). The second mechanism was first proved by
use of deuterated silica. 51 Indeed if the reaction proceeds via reaction with
the alkyl moiety, all neopentane should be monodeuterated while in the
second case only one fourth will be deuterated, as observed experimentally.
Later the pentacoordinated intermediate was observed by solid-state 13 C
NMR and its molecular analogue was synthesized confirming the above
mechanism. 52
In the case of alkylidynes complexes (for example W[CH 2 -C(CH 3 ) 3 ] 3 [ CH-
C(CH 3 ) 3 )] quite the same reactivity is observed with the formation of surface
alkylidynes species. 53 Another example is the rhenium complex Re[CH 2 -
C(CH 3 ) 3 ] 2 [ ¼ CH-C(CH 3 ) 3 )][ CH-C(CH 3 ) 3 )] which contains the three different
ligands and whose reaction product with silica dehydroxylated at high
temperature has been fully characterized by various physico-chemical
methods and DFT calculations (Figure 1.1). 54 For this surface complex the
presence of an agostic interaction between the metal and the hydrogen of
the carbyne moiety could be proved by solid-state NMR. This illustrates the
precision which can be attained in the characterization of a surface complex.
More recently, additional data on the nature of the bond between the
metal and the silica support were obtained by 17 O solid-state NMR. 55 Not
only the oxygen atom of the Si-O-M bridge could be identified but also the
presence of Si-O-Si bridges in small interaction with the metal was ob-
served giving new insights into the structure of the grafted organometallic
species and their interaction with the silica carrier.
This chemistry has then been extended to other complexes containing
'inert' ligands in combination with alkyl or alkylidene ones in view of the
synthesis, for example, of olefin metathesis catalysts. This is the case of a
tungsten imido complex based on the Schrock catalyst. 56,57 Another example

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